Effect of holding time on E.coli in wastewater samples

Changing the Paradigm for Water Pollution Monitoring
Oral Presentation

Prepared by , M. Citriglia

Contact Information: schafern@neorsd.org; 216-641-6000


The U.S. Environmental Protection Agency (USEPA) defined Escherichia coli (E. coli) as one of the best indicator r organisms to determine the water quality for a number of different water sources including drinking water, waste water effluents, and recreational water. The holding-time for analysis varies based on the type of water being analyzed. Non-potable waters have a holding time of 8-hours while finished potable water allows a holding time of 30-hours. The shorter holding time for non-potable waters can create problems for laboratories analyzing samples for compliance with a Combined Sewer Overflow (CSO) consent decree or additional CSO monitoring as part of an NPDES permit. The Northeast Ohio Regional Sewer District (District) is a large regional water reclamation facility and stormwater utility located in Northeast Ohio. In 2011 the District entered into a Consent Decree with the U.S. Department of Justice, USEPA, and Ohio Environmental Protection Agency to eliminate an estimated four billion gallons of CSO annually, and achieve 98% capture of CSO. The District is required to expand treatment, handle increased flow at all three facilities, along providing treatments to any bypass events. The consent decree and NPDES permit have included monitoring of discharges during these events. One of the major challenges the District had while determining the best way to comply with the CSO consent decree was when and how to sample whenever a wet weather event occurred. Since most wet weather events occurred outside the normal working hours the District decided to evaluate the use of extended holding for samples. Based on prior articles and white papers the District designed a study to determine the viability of longer holding-time of 24-hours or 48-hours instead of 8 hours for samples collected for bacterial analysis. The study included the collection of wastewater influent, primary effluent, final effluent, and final effluent subjected to chlorination and dechlorinating. Samples were collected and analyzed at 8, 12, 24 and 48 hours after collection and stored at <6.0°C. The samples were analyzed using the Collilert method and EPA method 1603. The data indicates that the holding time had significant reduction of bacterial count in case of raw influent and primary effluent. The samples collected at the treated effluent had a slight elevation in bacterial density however this was not determined to be significant. The samples collected from the raw influent and primary effluent had a significant decrease in bacterial density with the extended holding time. This presentation will outline the data from the holding time study, along with comparison of data between the Colilert and the EPA 1603 and how this data was used to make decisions on how best perform a two-year wet weather HRT design and implementation strategy.